1. Field of the Invention
The present invention relates to an apparatus for measuring the concentration of developer used in a liquid printer, and more particularly, to an apparatus for measuring the concentration of developer used in a liquid printer developer by forming a developer film and measuring the amount of light transmitted through the developer film.
2. Description of the Related Art
In general, a liquid electrophotographic printer is a device for forming an electrostatic latent image by selectively scanning a laser beam onto a photoreceptor medium such as a photoreceptor web or a photoreceptor drum and developing a toner image from the electrostatic latent image with developer which is a mixture of toner and a liquid carrier in a predetermined ratio.
A color liquid electrophotographic printer, as shown in FIG. 1, includes a plurality of development units 20 for developing an electrostatic latent image formed on the image recording surface of a photoreceptor web 1 which is one of photoreceptor media by a laser beam selectively scanned by a laser scanning unit (LSU) 10.
As shown in the drawing, the development units 20 are typically arranged to form a toner image in order of yellow (Y), magenta (M), cyan (C) and black (K) colors. Each development unit includes a container 21 for developer corresponding to the respective yellow (Y), magenta (M), cyan (C) and black (K) colors, and a developing portion 25 for receiving developer supplied from the container 21 by a developer supply apparatus 23 and for developing a toner image from the electrostatic latent image formed on the photoreceptor web 1.
The toner image developed at the developing portion 25 is dried at a drying unit 30 and transferred to a transfer roller 41. The transferred image is printed on a sheet of paper 45 supplied between the transfer roller 41 and the fusing roller 43. Any remaining developer after the development is collected in the container 21 through the developing portion 25.
Also, the liquid electrophotographic printer generally includes a concentration measuring apparatus 50 for measuring whether the concentration of the developer is appropriate. The concentration measuring apparatus 50 measures the concentration of developer by emitting light to the developer supplied to the developing portion 25 and measuring the light transmittance thereof.
However, as the toner image may not be completely transferred onto the transfer roller 41 during the transfer process, some toner may remain on the photoreceptor web 1. As the photoreceptor web 1 rotates, any remaining color toner is carrier along through the developing portions 25. In doing so, the remaining color toner may enter inside the developing portion 25 for developing a color different from the remaining color and be collected to the container 21 so as to contaminate the original developer color contained in the container 21. Also, even before the toner image reaches the drying unit 30, some of the developer adhering to the photoreceptor web 1 at the developing portion 25 enters the subsequent developing portions to contaminate the developers there.
When the developer is contaminated, the concentration measuring apparatus 50 detects a different concentration value from the actual concentration value due to the difference in the light transmittance of the different color toners. For example, in the case of yellow developer contaminated by a small amount of black developer, the concentration value detected by the concentration measuring apparatus 50 is greater than the actual value as the black contaminant causes the light transmittance of the yellow developer to sharply decrease. This is because the light color developers such as yellow, magenta and cyan have light transmittances of about 70-80% or more except for a particular wavelength range corresponding to the color of the developer, whereas black developer exhibits very low light transmittance with respect to light of most wavelengths.
Thus, when developer is contaminated by a different color, in particular, black toner, the concentration measuring apparatus 50 measures an incorrect concentration which is different from the actual concentration value. Also, if the sample of developer to be measured is thick and the developer is contaminated by black toner, the light transmittance approaches zero so that the concentration measurement cannot be performed.